Editor's note: Chemical Online is pleased to present this four-part article on orbital welding for bioprocess piping by industry expert Barbara Henon of Arc Machines. This article was adapted from a talk given late last year by Dr. Henon at an ASME meeting.
Preventing a loss of corrosion resistance. Highly purified water such as DI or WFI is a very aggressive corrodent for stainless steel. In addition, pharmaceutical grade WFI is circulated at elevated temperatures (80°C) to maintain sterility. There is a fine line between dropping the temperature low enough to support viable organisms which would be fatal to the product, and raising the temperature high enough to promote the production of "rouge". Rouge is a brownish film of variable composition resulting from corrosion of stainless steel piping system compo-nents. Dirt and iron oxides are likely to be principal components, but iron, chromium and nickel in various forms may also be present. The presence of rouge would be fatal to some products and its presence may result in further corrosion, although its presence in other systems appears to be fairly benign.
Welding can have a detrimental effect on corrosion resistance. Heat tint, which is the result of oxidized material precipitated on the weld and HAZ during welding, is particularly harmful and has been implicated in the formation of rouge in pharmaceutical water systems. The formation of chromium oxides, which contribute to the heat tint, leaves an underlying chromium-depleted layer which is vulnerable to corrosion. Heat tint can be removed by pickling and grinding which removes metal from the surface including the underlying chromium-depleted layer and restores the corrosion resistance to near base metal levels. However, pickling and grinding are detrimen-tal to the surface finish. Passivation of piping systems with nitric acid or chelant formulations is done to overcome the detrimental effects of welding and fabrication before the piping systems are placed in service. Auger electron analysis has shown that chelant passivation can restore the surface changes in the distribution of oxygen, chromium, iron, nickel, and manganese which oc-cur across the weld and heat affected zone to the preweld condition. However, passivation af-fects only the outer surface layer and does not penetrate below 50Å, while heat tint can extend 1000Å or more beneath the surface.
Thus, in order to have a piping system installed that approaches the corrosion resistance of the unwelded base material, it is important to attempt to limit the damage produced by welding and fabrication to a level that can be essentially restored by passivation. This entails using purge gas with minimal oxygen content and delivering it to the ID of the weld joint without contami-nation by atmospheric oxygen or moisture. Accurate control of heat input during welding and avoidance of excessive heat is also important to prevent loss of corrosion resistance. Control of the fabrication process in order to achieve repeatably consistent high quality welds, and careful handling of stainless steel tubing and components during fabrication to prevent contamination are essential requirements for achieving a high-quality piping system that will resist corrosion and provide a long productive service life.
The materials used for high-purity biopharmaceutical stainless steel piping systems have under-gone an evolution during the past decade towards improved corrosion resistance. Most stainless steel used prior to 1980 was 304 stainless steel as it is relatively inexpensive and was an im-provement over the previously used copper. In fact, 300 series stainless steel is comparatively easy to machine and can be fusion welded without excessive loss of its corrosion resistance and requires no special preheat nor postheat treatment.
More recently, there has been an upwards trend in the use of 316 stainless in high-purity piping applications. Type 316 is similar in composition to type 304 but in addition to the chromium and nickel alloying elements common to both, 316 contains about 2% molybdenum which sig-nificantly improves the corrosion resistance of 316. Types 304L and 316L, called "L" grades, were developed with less carbon (0.035% compared to 0.08%) than standard grades. This re-duction in carbon content was intended to reduce the amount of carbide precipitation that could occur as a result of welding. This is the formation of chromium carbide, which depletes the grain boundaries of the base metal of chromium and leaves it vulnerable to corrosive attack. The for-mation of chromium carbides, called "sensitization", is time and temperature dependent and was a much greater problem when welds were done manually. We have demonstrated that or-bital welding of a super-austenitic stainless steel, AL-6XN, provides significantly more corro-sion-resistant welds than similar welds done manually. This is because orbital welding provides precise control of amperage, pulsation and time resulting in much low-er, and more even heat input than manual welding. Orbital welding in combination with the use of the "L" grades of 304 and 316 has virtually eliminated carbide precipitation as a factor in the development of corrosion in piping systems.
Heat-to-Heat Variation in Stainless Steels. Although weld parameters and other factors can be held to fairly tight tolerances, there are still variations in the heat input required to weld dif-ferent heats of stainless steel. A heat number is the batch number assigned to a particular melt of stainless steel at the mill. The exact chemical composition of each batch is recorded on a mill test report (MTR) along with the identification or heat number for the batch. While pure iron melts at 1538°C (2800°F), alloyed metals melt over a range of tem-peratures depending upon the type and concentration of each al-loying or trace element present. Since no two heats of stainless steel will contain the exact same concentrations of each element, the welding characteristics will vary somewhat from heat to heat.
SEMs of orbital welds on 316L tubing done on AOD tubing (above) and EBR material (below) show considerable differences in smoothness of the weld bead.
Figures: Courtesy of Valex Corp.
While a single weld program may work for most heats of a similar OD and wall thickness, some heats will require less amperage, and some will require more am-perage than is typical. For this reason, careful track must be kept of different material heats on a jobsite in order to avoid potential problems. Usually, a new heat will require only slight changes in amperage to arrive at a satisfactory weld program.
The Sulfur Issue. Elemental sulfur is an impurity asso-ciated with iron ore and is largely removed during the steel manufacturing process. AISI type 304 and 316 stain-less steels have a specified maximum sulfur content of 0.030%. With the development of modern steel refining processes such as argon oxygen decarburization (AOD) and double vacuum melting practices such as vacuum in-duction melt followed by vacuum arc remelt (VIM+VAR) it has become possible to produce steels that are very specific in their chemical compositions. It has been noted that when the sulfur content of steel falls below about 0.008%, the properties of the weld puddle are changed. This has been attributed to the effect that sulfur, and to a lesser degree other elements, has on the temperature coefficient of surface tension of the weld puddle which determines the flow characteristics of the liquid puddle.
At very low sulfur concentrations (0.001% - 0.003%), the weld puddle becomes very wide with respect to the depth of penetration compared to a similar weld done on materials with interme-diate levels of sulfur. A weld done on a low-sulfur stainless steel tube will have a wider weld bead, and on thicker walled tubing (0.065 in., or 1.66 mm or greater), there will be a greater ten-dency to have a concave weld on the outside when the weld current is sufficient to produce a fully-penetrated weld. This makes the very low sulfur material harder to weld, especially with thicker tube walls. At the higher end of the sulfur concentration for 304 or 316 stainless steels, the weld bead tends to be less fluid in appearance and somewhat rougher than on materials in-termediate in sulfur. Thus for weldability, the ideal sulfur content would range from about 0.005% to 0.017% as specified in ASTM A270 S2 for Pharmaceutical Quality Tubing.
Producers of electropolished stainless steel tubing have noted that, with even intermediate lev-els of sulfur in 316 or 316L stainless steels, it is difficult to meet the demands of their semicon-ductor and biopharmaceutical clients for a smooth, pit-free inner surface. It is increasingly common to verify the smoothness of the tubing surface finish with a scanning electron micro-scope. Sulfur in the base metal has been shown to form non-metallic inclusions or manganese-sulfide (MnS) "stringers" which are removed during electropolishing and leave voids in the range of 0.25-1.0 microns.
The manufacturers and suppliers of electropolished tubing are driving the market towards the use of ultra-low sulfur materials to meet their surface finish requirements. However, the prob-lem is not limited to electropolished tubing, since in non-electropolished tubing, the inclusions are removed during passivation of the piping system. The voids have been shown to pit prefer-entially to smooth surface areas. Thus there are some valid reasons for the trend towards lower sulfur, "clean" material.
Arc Deflection. In addition to improving the weldability of stainless steel, the presence of some sulfur increases the machinability as well. Thus manufacturers and fabricators tend to select ma-terials at the higher end of the specified sulfur range. Welding of tubing with very low sulfur concentrations to fittings, valves or other tubing which have higher sulfur contents presents a welding problem, since the arc will deflect towards the tubing with the low sulfur content. When arc deflection occurs, penetration becomes deeper on the low sulfur side in comparison to the higher sulfur side, which is the converse of what happens when welding tubing of matching sul-fur concentrations. In an extreme case, a weld bead can fully penetrate the low sulfur material and leave the weld joint completely unfused on the inside (Fihey and Simeneau, 1982). In an attempt to match the sulfur contents of fittings to that of tubing, Carpenter Steel Division of Car-penter Technology Corporation in Pennsylvania has introduced a low sulfur (0.005% max.) 316 bar stock (Type 316L-SCQ) (VIM+VAR) for manufacturing fittings and other components in-tended to be welded to the low sulfur tubing. It is much easier to weld two heats of very low sulfur material to each other than to weld one that is very low in sulfur to one that is higher.
The shift towards the use of low sulfur tubing is largely driven by the need to achieve a smooth electropolished inner tube suface. While surface finish and electropolishability are important to both the semiconductor industry and the biotech/pharmaceutical industries, SEMI, writing specifications for the semiconductor industry, has specified that 316L tubing for process gas lines must have an upper limit of 0.004% sulfur for an optimum surface finish. On the other hand, the ASTM has modified their ASTM 270 specification by including a pharmaceutical grade of tubing which limits sulfur to a range of 0.005 to 0.017%. This should result in fewer welding difficulties than the lower range of sulfur. However, it should be noted that even within this limited range it is still possible to get arc deflection when welding the lower sulfur tubing to the higher sulfur tubing or fittings, and installers should carefully track material heats and check for welding compatibility between heats before making production welds.
Other Trace Elements. Trace elements including sulfur, oxygen, aluminum, silicon and man-ganese have been found to affect penetration. Trace amounts of aluminum, silicon, calcium, titanium, and chromium which are present as oxide inclusions in the base metal, have been asso-ciated with slag formation during welding.
The effects of the various elements are cumulative, so the presence of oxygen can offset some of the effects of low sulfur. The positive effects on penetration of sulfur can be offset by high levels of aluminum. Manganese volatilizes at welding temperatures and deposits in the weld HAZ. These manganese deposits have been implicated in a loss of corrosion resistance. (See Cohen, 1997). The semiconductor industry is currently experimenting with low-manganese and even ultra-low manganese 316L materials to prevent this loss of corrosion resistance.
Slag Formation. Slag islands occasionally appear along the weld bead of some heats of stain-less steel. This is essentially a material problem, but sometimes a change in weld parameters can minimize the condition, or a change to an argon/hydrogen gas mix may improve the weld. Pol-lard found that the ratio of aluminum to silicon in the base metal affects slag formation. To pre-vent the formation of undesirable patch-type slags, he recommended that the aluminum content be held to 0.010% for a silicon content of 0.5%. However, globular slags rather than the patch type may form when the aluminum/silicon ratio is above this level. This type of slag could leave pits after electropolishing which would be unacceptable for high-purity applications. Slag is-lands which form on the weld OD may result in non-uniform penetration of the ID weld bead and may cause a lack-of-penetration. Slag islands which form on the ID weld bead may be sus-ceptible to corrosion.
Single-pass weld with pulsation. The standard automatic orbital tube weld is a single-pass weld with pulsed current and continuous constant speed rotation. This technique is suitable for tubing from 1/8 in. to approximately 7 in. OD with wall thicknesses of 0.083 in. and under. After a timed pre-purge an arc is struck. Penetration of the tube wall is accomplished during a timed delay in which an arc is present, but rotation does not take place. After this rotation delay the electrode rotates around the weld joint until, during the last level of the weld, the weld ties-in or overlaps the initial part of the weld. When tie-in is complete the current is gradually ta-pered off in a timed downslope.
Step Mode ("Synchro" welds). For fusion welding of heavier-walled materials, generally with wall thicknesses greater than 0.083 in., the fusion welding power supplies may be used in the syn-chro or step mode. In synchro or step mode, the welding current pulsation is synchronized with the travel so that the rotor is stationary during the high current pulse to achieve maximum pen-etration, and moves during the low current pulse. The synchro technique uses much longer pulse times, on the order of 0.5 to 1.5 seconds compared to tenths or hundredths of a second pulse times for conventional welds. This technique is effective for welding of thin-walled pipe up to about 2 in. schedule 40 which has a wall thickness of 0.154 in., or 6 in. schedule 5. The step technique produces a wider weld bead which makes it forgiving and helps to weld irregular pieces such as fittings to tubes where there may be some difference in the dimensional tolerances between the tubing and fitting, some misalignment, or incompatibility of material heats. This type of weld takes about twice as much arc time as a conventional weld and because of the wider, somewhat rougher weld bead, is less suitable for ultra-high-purity (UHP) applications.
Programmable Variables. The present generation of welding power supplies are micro-processor-based and store programs which specify nu-merical values of weld parameters for a specific diameter (OD) and wall thick-ness of the tube to be welded including purge times, welding currents, travel speed (RPM), number of levels and time for each level, pulsation times, downslope time, etc. For orbital pipe welds with filler wire addition, program parameters would include wire feed speed, torch oscillation amplitude and dwell times, AVC (arc voltage control to provide a constant arc gap), and upslope. In order to make a fusion weld the weld head, with proper electrode and tube clamp inserts installed, is mounted on the tube and the weld schedule or program is called up from the power supply memory. The weld sequence is initiated by pressing a button or membrane panel key and the weld proceeds without operator intervention.
Non-Progammable Variables. In order to get consistently good weld quality, the weld param-eters must be carefully controlled. This is accomplished by the accuracy of the welding power supply and the weld program, which is the set of instructions entered into the power supply con-sisting of weld parameters for welding a particular size of tube or pipe. There must also be a set of weld standards in effect which specifies weld acceptance criteria and some system for weld inspection and QC to assure that the welds meet the agreed upon criteria. However, certain fac-tors and procedures other than the weld parameters must be carefully controlled as well. These factors include the use of good end-preparation equipment, good cleaning and handling practic-es, good dimensional tolerances of the tubing or other components being welded, consistent tungsten type and dimensions, highly purified inert gas, and careful attention to changes of ma-terial heats.
The requirements for preparation of the tubing ends for welding are much more critical for or-bital welding than for manual welding. The weld joint for orbital tube welding is typically a square butt joint. A precise, consistent, machined end-preparation is needed in order to attain the repeatability that is expected from orbital welding. The end must be squared with no burrs or bevel on the OD or ID (outside or inside diameter) which would cause a difference in the wall thickness, since welding currents are based on wall thickness.
The tubing ends must fit together in the weld head so that no visible gap is apparent between the two ends of the square butt joint. Although it may be possible to complete a weld joint with a small gap, the weld quality may be adversely affected. The larger the gap, the more likely it is that there will be a problem. Poor fit-up can result in complete failure to make the weld. Tubing saws made by George Fischer and others which cut the tubing and face the tube ends in the same operation, or portable end-preparation lathes such as those made by Protem, Wachs, and others, are commonly used to make smooth machined ends suitable for orbital welding. Chop saws, hacksaws, bandsaws, and tubing cutters are not suitable for this purpose.
In addition to the weld parameters that are entered into the power supply to make a weld, there are other variables which can have a profound effect upon the weld, and yet are not part of the actual weld program. These include the tungsten type and dimensions, the type and purity of gas used to shield the arc and to purge the inside of the weld joint, the gas flow rates used for purging, the type of weld head and power supply used, joint configuration, and any other rele-vant information. We call these the "non-programmable" variables and record them on the weld schedule sheet. For example, the type of gas is considered to be an essential variable for the weld procedure specification (WPS) done to qualify a welding procedure to ASME Section IX of the Boiler and Pressure Vessel Code. A change in the type of gas or a change in percentage of a gas mixture, or the elimination of an ID purge requires that the welding procedure be requalified.
Welding Gas. Stainless steel is resistant to oxidation by atmospheric oxygen at room tempera-ture. When it is heated to the melting point, (1530°C or 2800°F for pure iron) it is highly subject to oxidation. Inert argon gas is most commonly used for shield gas as well as for purging the interior weld joint with the orbital GTAW process. The purity of the gas with respect to oxygen and moisture determines the amount of discoloration due to oxidation that appears on or near the weld after welding. The oxidation may be a light tea color, or a faint blue if the purge gas is not of the highest quality or if the purging system is not entirely leak-free so that trace amounts of air leak into the purge system. The absence of any purge, of course, results in a black crusty surface commonly referred to as "sugaring." Welding grade argon which is supplied in cylin-ders has a purity of 99.996-99.997% depending on the supplier with 5-7 ppm oxygen and other impurities, which would include H20, 02, C02, hydrocarbons, etc., totaling 40 ppm as a maxi-mum. High purity argon in cylinders or liquid argon in dewars can have a purity of 99.999% or a total of 10 ppm impurities with a maximum of 2 ppm oxygen. Note: Gas purifiers such as the Nanochem or Gatekeeper can be used during purging to bring contaminant levels down to the low parts per billion (ppb) range.
Mixed Gases. Gas mixtures such as 75% helium/25% argon and 95% argon/5% hydrogen may be used as shield gases for special applications. Both of these mixtures produce a hotter weld than one done at the same program setting as with argon. The helium mixture is especially useful for achiev-ing maximum penetration with fusion welds on car-bon steel. A consultant to the semiconductor industry has promoted the use of the argon/hydrogen mixture as a shield gas for UHP applica-tions. The hydrogen mixture offers several advantages but also some serious disadvantages. The advantages are that it produces a wetter puddle and smoother weld surface which is desir-able for achieving a UHP gas delivery system with as smooth an inner surface as possible. The presence of hydrogen provides a reducing atmosphere so that if oxygen is present in trace amounts in the gas mixture, the resulting weld appears cleaner with less discoloration than with a similar oxygen concentration in pure argon. This effect is optimum at about 5% hydrogen. A 95/5% argon/hydrogen mixture has been used by some as an ID purge to improve the appear-ance of the inner weld bead.
The weld bead with the hydrogen mixture used as a shield gas is narrower except with very low sulfur-containing heats of stainless steel, and more heat is produced in the weld than at the same amperage settings with unmixed argon. A noticeable disadvantage of the argon/hydrogen mix-ture is that the arc is considerably less stable than with pure argon, and there is a tendency to get arc wander that can be severe enough to result in lack-of-fusion. Arc wander may disappear when a different source of mixed gas is used suggesting that it may result from contamination or poor mixing. Since the amount of heat produced by the arc varies with the hydrogen concen-tration, a constant concentration is essential in order to achieve repeatable welds and there is variability in pre-mixed bottled gas. Another disadvantage is that tungsten life is considerably shorter when a hydrogen mixture is used. Although the cause of tungsten deterioration with the mixed gas has not been determined, it has been reported that arc strike is more difficult and the tungsten may need replacement after only one or two welds. Argon/hydrogen mixtures can not be used for welding carbon steel or for titanium.
A significant feature of the TIG process is that the electrode is not consumed. Tung-sten has the highest melting point of any metal (6098°F; 3370°C) and is a good emitter of electrons making it especially suitable for a non-consumable electrode. Its properties are improved by the addition of 2% of certain rare earth oxides, such as ceria, lanthana or thoria, improving arc strike and arc stability. Pure tungsten is seldom used for GTAW because ceriated tungstens have superior properties, especially for or-bital GTAW applications. Thoriated tungstens are used less often than in the past because they are somewhat radioactive.
Electrodes with a ground finish are dimensionally more uniform. A smooth finish is always preferable to a rough or inconsistent finish, since consistency in electrode geometry is essential for consistent uniform weld results. Electrons emitted from the tip (DCEN) transfer heat from the tungsten tip to the weld. A finer tip permits the current density to be maintained at a very high level but may result in shorter tungsten life. For orbital welding, it is very important for the electrode tip to be machine ground to assure repeatability of the tungsten geometry and thus of the welds. A blunt tip forces the arc to originate at the same place on the tungsten from weld to weld. The tip diameter controls the shape of the arc and the amount of penetration at a par-ticular current. Taper angle affects current/voltage characteristics of the arc and must be spec-ified and controlled. Tungsten length is significant because one can use tungsten of known length to set the arc gap. The arc gap at a particular current value determines the voltage and hence the power applied to the weld.
The electrode size and its tip diameter are chosen based on welding amperage. If the current is too high for the electrode or its tip it may lose metal from the tip whereas using an electrode with a tip diameter too large for the current may cause the arc to wander. We specify electrode and tip diameters by wall thickness of the weld joint and use 0.0625 diameter for practically eve-rything under 0.093 in. wall except when using the mini-heads (Model 9-500 and Model 9-250) which were designed to be used with 0.040 in. diameter electrodes for welding small delicate parts. For repeatability of the weld process, the tungsten type and finish, the length, taper angle, diameter, tip diameter, and arc gap must be all be specified and controlled. For tube welding applications, ceriated tungsten is always recommended, as this type exhibits substantially long-er lifetime than other types and has excellent arc ignition characteristics. Ceriated tungsten is non-radioactive.
To see the previous installments of this article, follow these links:
I. Considerations For Orbital Welding In BioProcess Piping Applications
For more information: Barbara Henon, manager, Technical Publications, Arc Machines, Inc., 10280 Glenoaks Blvd., Pacoima, CA 91331. Tel: 818-896-9556. Fax: 818-890-3724.